1. Field of the Invention
The present invention relates to a method employing analog digital conversion of color television signals for increasing the signal to noise ratio during transmission of such a signal.
2. Brief Description of the Background of the Invention Including Prior Art
It is known that for color television the reproduction of the picture employs three signals which are transmitted, that is, the capatible luminance signal Y and the two color difference signals B-Y and R-Y. In commercial color television emission the luminance signal Y is transmitted by amplitude modulation of a picture, and the two color difference signals are transmitted by quadrature modulation of a color carrier.
If a digital signal transmission is performed within a transmission path then the three analog signals have to be converted into digital signals with analog to digital converters. An example for such a digital signal transmission is the so-caleed timeplex system for the transmission or recording of color television signals as it is described for example in "Fernseh- und Kino- Technik" 1983, pp. 187-196 and in particular, on pp. 193 and 194. All signals are time compressed for a transmission or recording and after the transmission and recording they are time expanded according to this system. The analog to digital conversion and digital to analog conversion is required, since the clocked memory storage serving for time compression and time expansion processes digital signals. According to the timeplex system alternatingly from line to line on the time compressed color difference signal B-Y and the time compressed luminance signal and on the other hand the time compressed color difference signal R-Y and the time compressed luminance signal Y are transmitted or recorded.
The signals Y, B-Y, R-Y are independent of each other in their amplitudes and therefore they are not to be mixed. Therefore in principal three separate analog to digital converters are required for the analog to digital conversion. Because of the different amplitudes of the signals the required modulation range of the converters cannot always be fully employed such that in particular in the case of small signal amplitudes, a small signal relative to noise is generated.